The present invention relates to techniques for frame detection and word alignment for SONET and SDH data, and more particularly, to detection frame boundaries and to aligning the internal data path to the word boundary of data in a SONET or an SDH standard.
Synchronous optical network (SONET) and Synchronous Digital Hierarchy (SDH) are standards for data transmission across optical telecommunications networks. Data bits that are transmitted according to a SONET or an SDH standard are grouped into frames. Each frame contains numerous data bytes. The data bytes typically contain 8 data bits. The data bytes are typically grouped into words for internal processing. Any number of bits and bytes can form a word (e.g., 64 bits/8 bytes per word).
Each frame contains an overhead section and a payload section. The overhead section includes bits specified by a SONET protocol. The payload section includes the data bits that are being transferred from one location to another.
SONET uses a basic transmission rate of STS-1 that is equivalent to 51.84 Mbps. Higher-level signals are integer multiples of the base rate. For example, STS-3 is three times the rate of STS-1 (3×51.84=155.52 Mbps). An STS-12 rate is 12×51.84=622.08 Mbps.
STS-1 is a specific sequence of 810 bytes (6,480 bits), which includes various overhead bytes and an envelope capacity for transporting payloads. It can be depicted as a 90-column by 9-row structure. With a frame length of 125 microseconds μs (8,000 frames per second), STS-1 has a bit rate of 51.840 Mbps. The order of transmission of bytes is row-by-row from top to bottom and from left to right (most significant bit first).
The first three columns of the STS-1 frame are for the transport overhead. Each of the three columns contains 9 bytes, for a total of 27 bytes. Of these, 9 bytes are overhead for the section layer, and 18 bytes are overhead for the line layer. The remaining 87 columns constitute the STS-1 envelope capacity (payload and path overhead).
A receiver does not necessarily know where words or frames begin and end when it receives a serial stream of SONET data. Therefore, it would be desirable to provide techniques for aligning the data path inside a receiver to the word boundary of an input data stream.